It is generally known that a liquid crystal display device (LCD) includes a liquid crystal cell and a polarizing plate and requires a bonding layer or an adhesive layer to combine the above elements.
Such a polarizing plate may include a polarizer (sometimes called ‘polarizing film’) based on polyvinyl alcohol (PVA), which is elongated in a certain direction and contains an iodine compound or dichroic polarizing material adsorbed thereto and oriented, as well as a polarizer protective film laminated on opposite sides of the polarizer to protect them. More particularly, the polarizer may have a multi-layered structure in that a polarizer protective film based on triacetyl cellulose (TAC), an adhesive layer to be in contact with a liquid crystal cell on the protective film and a release film are formed at one side of the polarizer, while another polarizer protective film and a surface protective film comprising another adhesive layer laminated on a substrate film are formed at the other side of the polarizer.
When the polarizing plate with such a structure described above is fixed to the liquid crystal cell, the release film is stripped from the adhesive layer. The surface protective film may also be removed when, a role thereof is completed during post treatment. The release film and the surface protective film are generally formed using plastic materials, and therefore, may have improved electrical insulating properties and generate static electricity when the film is released from the polarizing plate.
The generated static electricity may cause several problems such as adsorption of foreign materials to an optical element causing contamination of a surface thereof, distorted orientation of liquid crystals causing staining on the optical element, damage of a thin film transistor (TFT) line, and so forth. In recent years, since a dimension of a polarizing plate used for fabrication of LCDs is enlarged with an increase in size of an LCD panel and generation of static electricity is significantly increased during high speed processing, it may be important to solve such problems.
In order to overcome electrostatic generation problems, a method for imparting anti-static properties to an adhesive has been proposed. For instance, addition of conductive materials comprising conductive metal powder or carbon particles to an adhesive or addition of ionic or non-ionic materials in a form of a surfactant has been employed. However, in order to impart anti-static properties, an excessive amount of conductive metal powder or carbon particles must be added and this may cause a problem of deteriorated transparency. Also, the surfactant is liable to be influenced by humidity and may transfer to a surface of the adhesive, causing a problem of lowered adhesive properties.
Japanese Patent Laid-Open No. 1993-140519 which was opened on Jun. 8, 1993, discloses a method for inhibiting generation of static electricity by adding a plasticizer based on ethyleneoxide modified dioctylphthalate to an adhesive to impart flexible properties thereto. However, this method entailed a problem of transferring to a surface of a polarizing plate which in turn has difficulty in inhibiting initial static electricity.
Korean Patent Laid-Open No. 2004-0030919 which was opened on Apr. 9, 2004, discloses a method for fabrication of an adhesive layer with a surface specific resistance of not more than 10—Ω/sq by adding an organic salt to prepare a pressure sensitive adhesive and using the adhesive. However, this method encounters a problem of consuming a large amount of expensive organic salts.
Japanese Patent Laid-Open No. 1994-128539 which was opened on May 10, 1994, discloses a method for imparting anti-static properties by combining polyetherpolyol with at least one alkali-metal salt. However, this method has problems in that an isocyanate based crosslinking agent may influence crosslinkage ability and surface transition due to hydrophilic properties of ethyleneoxide and thus may cause reduced adhesive properties.